Fixing holes in meshes

hello i have a shell on rhino (open mesh) that i created on grasshopper and baked on rhino - i tried multiple ways in order to make it solid (in order to be able to print it in a 3d printer without supports) and with a base of at least 5mm ffor each piece, but with no success for now…can someone please help me figure out how to make it? i tried also to exrude the meshes in 5mm but i still have ‘holes’ in the meshes. i need it to be total solid in the bottom but without changing the ‘hills topography’ at the top

i am attaching the rhino file
thanks a lot in advance
suprposition heatmap and topography - for 3d printer.3dm (6.7 MB)